The present disclosure relates generally to facsimile transmission through a packet switched network, and more particularly to compensation for network operations related to such facsimile transmissions.
Facsimile document transmission continues to have an important role in business communications for a number of reasons, including the ability to transfer images not stored on a local computer, legal acceptance of handwritten signatures, real-time confirmation of receipt, confidence in what has been sent/received, and the ability to provide a ‘tamper resistant’ copy of the information transferred. The ubiquitous nature of facsimile-enabled devices on a global scale allows them to easily take advantage of existent telecommunications networks. Such devices also may be shared by a number of individuals so that sending and receiving documents can be relatively efficient among a general population or group of persons.
While facsimile communications have previously been implemented over circuit switched networks, such as the publicly switched telephone network (PSTN), packet switched networks, such as Internet Protocol (IP) networks, have been implemented to carry communications including facsimile communications. As these different types of networks continue to coexist, translation and communication between them has become (and should continue to be) an important part of communications, including facsimile communications.
IP networks are inherently asynchronous, have a higher delay, and are relatively ‘lossy’ (lose or drop packets) compared to PSTN networks, which typically operate on a time-division multiplexed (TDM) basis. While these characteristics of IP networks are known to adversely impact both voice and facsimile communications, the impact to facsimile communications is typically more pronounced. Various solutions have been provided to overcome drawbacks related to IP network communications; however, they tend to be focussed on voice data and in many cases can cause more problems than they solve. Facsimile users thus tend to have a negative experience when attempting to perform voiceband (non-T.38) facsimile transmissions over packet switched networks.
Translation between circuit switched and packet switched communication networks typically involves the use of translation between different protocols, and is often performed by gateways, sometimes referred to as IP media gateways. A gateway can carry different types of communications between various network types, such as an IP network and a PSTN. Such different types of communications may include voice or facsimile, for example. The gateway typically provides protocol translation service between the networks for these different types of communications. Facsimile transmissions typically adhere to the International Telecommunication Union (ITU) T.30 specification, and are often implemented using the realtime facsimile transmission specification under ITU T.38.
One or more of the nodes in an IP network may not support real time facsimile protocols such as the T.38 protocol or may have interoperation issues with the protocols. In such case(s), the IP network typically relays the realtime facsimile messages using a facsimile pass-through technique that involves other types of protocols and codecs for handling facsimile transmissions originating from PSTN 112. Currently, G.711 (64 kbps) and G.726 (32 kbps) codecs are commonly used facsimile pass-through codecs and are well suited for facsimile transmission due to the low compression levels involved in implementing the codecs. The G.711 codec is often used as a default for pass-through facsimile transmissions, since it is supported in VoIP implementations. The low compression levels of the G.711 codec make it possible for facsimile modem data to be preserved through the compression process with sufficient integrity to permit successful facsimile transmission. The IP network pass-through mode operates similarly to a PSTN-based facsimile transmission once a VoIP G.711 call is established. When the G.711 codec is used to pass a facsimile transmission through the IP network in pass-through mode, the various network nodes, including gateways, generally do not distinguish a facsimile call from a voice call.
When transmitting voice communications, gateways typically support VoIP and can take advantage of voice activity detection (VAD) during voice calls to reduce bandwidth utilization in the IP network. In such a scenario, voice conversation transmissions can readily take advantage of VAD to reduce bandwidth usage that is used to carry voice data, and to avoid carrying communication transmissions that have silence for voice data. This type of silence suppression substitutes “silence” packets for non-speech packets to avoid sending packets that might amplify noise picked up during transmission. Thus, active voice conversations can be carried without also carrying non-speech data, which in turn permits a reduced bandwidth usage for voice conversation type communications to enable communication networks, such as the IP network, to operate more efficiently.
Silence suppression or VAD have the potential to cause corruption of facsimile data if valid facsimile signals become suppressed when they are detected as noise instead of voice communications in facsimile pass-through calls. For example, silence suppression or VAD can contribute to signal clipping, which can negatively impact modem data being transported in the communication network. Accordingly, facsimile pass-through calls are typically provided without engaging the features of silence suppression or VAD.
Packet switched networks can convey facsimile transmissions, such as by providing facsimile over IP (FoIP) service at the various nodes of the network that the facsimile transmission traverses. The nodes of the network may have different data rates for transmissions, due in part to differences in clocking frequency sources. Because of the discrepancy in clocking frequencies among different nodes of the network, certain nodes participating in a facsimile transmission may have an excess or shortage of data packets, such as real-time transport protocol (RTP) packets, during the transmission. The discrepancy in data rates between nodes of the communication network is sometimes referred to as clock skew, and can result in facsimile transmissions becoming distorted, slowed, or dropped when timing specification thresholds are not met due to the effects of clock skew.
FoIP calls may fail because of the lack of clock synchronization, e.g., clock skew, between peer voice gateways or between voice gateways and FoIP endpoints. Voice gateways are typically timed or clocked from local TDM sources, service providers or internal oscillators. FoIP endpoints use a variety of clock sources, which may include operating system timers and various PC hardware clocks. The effect of clock skew can be seen in an excess of RTP packets or as a shortage of RTP packets at a terminating gateway or at an FoIP endpoint. One technique for compensating for clock skew is to provide a common clock source for the digital signal processors (DSPs) in each peer gateway. However, such a technique can be complex and may necessitate the use of additional equipment that can be prohibitively expensive.
In a packet switched network, individual blocks of data are transported with varying propagation delay depending upon the route taken and network conditions at the time, sometimes referred to collectively as “jitter.” Jitter can be compensated at a receiving end or midpoint of a network transmission path by providing sufficient overall throughput delay to accommodate the range of propagation delays, often implemented with a jitter buffer in a network component such as an IP media gateway. Individual packets that have been delayed sufficiently to fall outside of a range that can be accommodated by a given jitter buffer are considered lost or dropped. The size of the jitter buffer is an important design consideration in constructing network components or networks in general. For example, a network component that implements a relatively large jitter buffer, with an attendant large overall delay, provides a greater tolerance to jitter and packet delays. However, if the jitter buffer size provides a significant overall delay, the result can be uncomfortably long pauses which can cause both parties to attempt to speak at the same time.
To address these competing objectives, many jitter buffers are adaptive, and dynamically vary their size to minimize the delay according to current network conditions (adaptive jitter buffers). Changing the size of a jitter buffer involves inserting or discarding data, which itself is likely to introduce distortions.
The algorithms used to perform adjustments to a jitter buffer size and/or delay are typically optimized for perceived voice quality. However, modem communications, including facsimile communications, are much less tolerant of the changes that adjustments to jitter buffer size can introduce in overall and round-trip delay, particularly with the use of echo cancelling type modems (e.g., V.34 protocol modems). Modems are also less tolerant of the introduced distortions caused by the step changes in jitter buffer size, which for facsimile transmissions can typically result is some distortion in the received image (or call elongation as image fragments are re-transmitted). Because of these issues that can arise when facsimile transmissions are carried over a packet switched network, many network components are configured to disable adaptive jitter buffers for facsimile and modem communications, and instead are configured to set a fixed jitter buffer size for the duration of such calls.
Facsimile transmissions can tolerate a relatively high overall delay in comparison to voice transmissions. However, when there is significant delay present, particularly when accumulated over multiple devices or network components, facsimile transmissions can fail due to the round trip delay exceeding T.30 timeout values. In addition, failures can occur when the communication delay is greater than that which a typical PSTN facsimile device, such as facsimile device 110, is expected to encounter and handle. Clock skew tends to exacerbate these failures because of the lack of synchronization. The receiver can remove or insert data at various intervals, such as periodically, in an effort to re-synchronize network nodes. As in the case of changing the size of adaptive jitter buffers to accommodate varying line conditions, such action introduces distortion and changes in the overall delay. This condition is true even in solutions that utilize a fixed, large jitter buffer.
When transmitting voice data, a gateway can typically re-synchronize the jitter buffer with other network components during periods of silence. Periods of silence are often available during voice transmissions, since such transmissions tend to be half-duplex in nature, and silence suppression can be used to reduce bandwidth for the call.
When transmitting facsimile data in pass-through mode, a gateway often does not have an opportunity to re-synchronize with other network components during periods of silence. This lack of opportunity to re-synchronize often occurs because facsimile pass-through applications disable silence suppression for the duration of a facsimile call. Moreover, if the jitter buffer of a gateway attempts to compensate for clock skew by removing or creating extra silence zones in the middle of fax image or command data, the facsimile data experiences amplitude or phase shifts that typically either causes the facsimile modem to train down or to drop the facsimile call all together. Training down refers to a reduction in a facsimile transmission rate, where the rate is reduced to a next lower available rate that the endpoint facsimile devices can accept. An FoIP endpoint is not typically constrained by the same real-time timing limitations as voice gateways and may sometimes have greater flexibility than the gateways in handling facsimile communications.
It would be desirable to overcome the drawbacks related to facsimile transmissions in a packet switched network that employs pass-through mode, including the drawbacks associated with fixed-length jitter buffers, clock skew and disabled silence suppression, which can undermined the quality or success of an FoIP transmission.